The Grant supports a general program of research into the physical properties of spin labels used as probes of structure and function of biological systems. The advanced electron spin resonance (ESR) techniques of ENDOR, ELDOR, and Saturation Recovery are being applied to nitroxide radicals to determine (1) the spin-spin relaxation time (T2), (2) the spin-lattice relaxation time (T1e), (3) the nitrogen nuclear relaxation time (T1N), and (4) intramolecular motions. In the next grant year particular emphasis will be on methodology and theory of Saturation Transfer Spectroscopy at Q-Band. In addition, a major effort will be to explore the physics and biological applications of the lanthanide probe-spin-label method.